'06 CC: truck's lights all okay.
trailer lights seem to have filament reversed, as in
running light is bright and brake/turn is dim.
both sides.
symptoms are the same on trailer and new magnetic spare set.
the 2 fuses under the hood are good.
do not see any fuses under the dash.

I find that the ground lead on the 7-lead socket has no continuity with the frame or tow hitch. I assume there should be.
Is there a fuse for the ground lead?
Any reason not to simply directly ground that lead locally, perhaps with a fusible link, rather than hunting for the break?